A suitable device is needed for the direct measurement of oxygen partial pressure (PO2) in blood and tissue for both clinical and research applications. Methods currently available for measuring PO2 lack convenience, reliability, speed, and relevance to many situations of interest. Efforts to develop electrical sensors have not been successful. It is desirable to have a very small PO2 sensor which can be inserted into a blood vessel or tissue with little disturbance, and which will provide instantaneous and current PO2 monitoring for either short or extended periods of time. A fiber optic sensor is ideal for this application, with the advantage, for physiological use, of very small size and flexibility, safety, and low cost. A PO2 sensor has been developed, based upon the principle of fluorescence quenching by oxygen. The feasibility of the sensor and its satisfactory performance has been demonstrated in in vitro and in vivo tests, in work in preceding years. In the previous year, the development of the sensor as a needle probe was accomplished, and problems of storage life and stability were resolved. During the past year, several problems relating to development of a usable instrument and probe were solved, and the system was used for experiments to measure PO2 in the eyes of dogs. This was done in comparison with polarographic electrode measurements of PO2. The experiments were done to evaluate the suitability of the sensor for studies of diabetic retinopathy.